HI,
can any one tell me the proper procedure for finding the instant center on a 1967 chevelle?
I’ve looked at many web sights and they tell me what the (IC) is and what goes in to checking the (IC) but no one tells me how to properly find the (IC) on my car?
Thanks

I like to start by fiding the actual center of gravity using a floor jack or two and a long section of 4x4 across the car between the frame rails. Jack the car in different areas until you find the point were the car will balance on the jack(s), you need only jack the car an inch or so off the ground to do this and be very careful not to let the car slide of the jack(s). Then for reference put a piece of masking tape vertically on the car at the balance point. Next tape a horizontal line on the side of the car, this line should run parrallel to and at the same height as the center of the camshaft. Where these two lines intersect is the approximate actual center of gravity.Now with the car on the ground and ballasted with your weight in the driver seat, tape a line on the car that is parrallel to and at the same height as the lower rear control arm. And another that is parralel to and at the same height as the upper rear control arm. These two lines will intersect each other at some point. Tape a vertical line on the car at the intersection point. Where the vertical line intersects the horizontal line (the camshaft line) is the approximate instant center of gravity.

I didn't think the instant center had anything to do with the center of gravity of the vehicle, the centroid of the mass or whatever you want to call it. My understanding is that it is the theoretical pivot point of the rear suspension which is found as Gary described, by extending the lines of the rear upper and lowers. Am I wrong?

Here's something with pictures.
http://www.performancetrends.com/4link.htm

kboorman, you're right IC is not a function of CG, I just like to know where CG is to help me understand mechanical advantage of IC relative to CG. You also don't want your IC behind CG.

Originally posted by Gary at GMPP:
You also don't want your IC behind CG. I know that would ruin MY day! :D ;)

Well there seems to be alot of differing opinions on which is the corect way to find the (IC)of my chevelle? I have seen the link to 4-Link Calculator software and have read it but it still does not tell on the demo drawing of how they got the (IC)? theres just an X on the side of the car where it is? Does any one have a copy of this software that I could have them run my car and I'll gladly pay them for it?
Thanks

http://www.geocities.com/kdslaby/Tech_Page_Glossary.html

The "backyard" approach that I use is:

When the vehicle is on a flat-level surface. Measure from centerline of tire to wheel well.
Place jackstands under each corner of car (as close to tires as possible. Verify that dimensions didn't change. Adjust as necessary to get back to normal ride height.
Take a framing square and place it in front of the differential and on the ground. You may need to extend the vertical leg, depending on how high the car is off the ground. Without moving the square,Measure from your vertical edge to the centeline of axle, centerline of upper mount, and lowermount bolt. You want both the height from ground (adjusted later) and distance to vertical edge. Repeat for forward control arm mounting positions. (It helps to clamp the framing square to a heavy piece of angle, so it doesn't move, while taking dimensions. If the square moves, start over.) Throw one of those angle finding levels on each rear control arm as a double check. Lower the car and measure from centerline of each axle to the ground.
NOw, with a fist full of scribbles, get a piece of graph paper. Place the graph paper horizontally (landscape). Draw a horizontal line ( I call it Zero)near the bottom and a vertical line (Axle centerline)near one side of the paper.
You can take either the overall length of the car or wheelbase and divide the number of square on the graph paper, so that each square is worth so much distance. Technically, you could also tape pieces together and use each square as a quarter inch. Transer all your dimensions to this paper as you have them. Take your axle to ground dimension and tranfer it to the paper. That is your "groundd line". You should now be able to circle your axle, draw in your tires, draw lines from rear control arm mount, through your square on the paper to the front arm. A protractor will reveal a number extremely close to the angles you found on the car. Now extend your lines.
You could have do it in the time it took to type this! If you have a 4 post lift, its even easier. Drop a plumb line.

Just a word of caution. You want to be able to repeat dimensions. If the tire pressure is uneven, or if the spring is sacked on one corner, or some other reason, the car is sitting askew, you will have a much harder time finding the instandt center. You may have to do both sides to arrive at a number. Also, don't forget to measure off ONE side of the square, or else draw the square to scale on the paper. Your distance between control arms where it mounts to the axl and frame should be verified to make sure you are logging the right dimensions. Garbage in...

Good luck !